Determination of Total Alkalinity of Soda Ash


Soda ash is the common term for commercial grade sodium carbonate. This material is used as a neutralizing agent in a variety of processes and products. In this exercise the student will determine the total neutralizing power of a soda ash sample, expressing the in terms of sodium oxide. The course of the titration will be followed potentiometrically. A secondary goal of the exercise is to allow the student to experimentally generate a titration curve for a polybasic substance. Comparison of that curve with the theoretical models may be informative.


Students will work in pairs, sharing the standardization of a single solution of HCl and in the potentiometric titrations. Each student will have a unique unknown.


Preparation and Standardization of HCl


Prepare an approximately tenth molar solution of hydrochloric acid following the details found in the text, section 37C-3, Preparation of Dilute HCl.


Standardize the hydrochloric acid solution following the details found in the text, section 37C-6, Standardization of HCl with Sodium Carbonate.



Potentiometric Determination of Soda Ash


General background on potentiometric titration is covered in Chapter 21 of Skoog, West, Holler, and Crouch, Fundamentals of Analytical Chemistry, 8th Ed., especially section 21G.


Connect the Vernier Lab-Pro system to laboratory computer and pH sensor to Lab-Pro interface. File pathway: Experiment Probes & Sensors pH system pH titration


Calibrate the pH electrode using pH 7 standard buffer.



Trial Titration The purpose of the trial titration is to familiarize the analyst with the procedure, operation of the software, and allow the analyst to estimate the approximate amount of titrant that will be necessary.


Weigh accurately by difference a dried sample of unknown soda ash (0.2 to 0.3 g) and add it to a 250 mL beaker containing a magnetic stirring bar. Add approximately 50 mL of water in the beaker, start the magnetic stirrer, immerse the pH electrode in the solution taking great care that the stirring bar does not hit the electrode assembly.


Titrate with HCl, taking pH readings every 1 mL. From the graph of pH vs volume HCl, identify the endpoint pH.



Final Titration Weigh accurately by difference a second sample of soda ash unknown (0.2 to 0.3 g) as before. From the information in the trial titration, estimate the volume of titrant that will be necessary for the mass of unknown used in this titration. Titrate as before, taking readings at 1 mL intervals, until within approximately 2 mL of the expected endpoint. From this point, take readings at small volume steps (approximately 0.1 mL).


From the pH and volume data, prepare ΔpH/Δvol vs volume plots to determine the effective endpoint. Calculate the results in terms of percent of Na2O in the unknown. Include the pH vs volume and the ΔpH/Δvol vs volume graphs with the lab notes.